Extraneuronal catecholamine in the iris of the rat: A consequence of nonsynaptic neurotransmission?

Abstract

Combined formaldehyde-induced fluorescence and radiochemical studies were performed to estimate the concentration of extraneuronal noradrenaline in the iris of the rat. Scanning microfluorimetric measurements showed that sympathetic denervation resulted in a decrease of extraneuronal fluorescence to65±5% of that of untreated controls. In vitro incubation of these denervated irides in the presence of various concentrations of noradrenaline resulted in an increase in extraneuronal fluorescence. A close linear relationship was found between the amount of unmetabolized noradrenaline taken up and the intensity of formaldehyde-induced fluorescence. After 1h of incubation in the presence of5 × 10 −6m noradrenaline, the extraneuronal fluorescence in denervated irides was restored to the level found in untreated rats. Based upon the amount of noradrenaline taken up under these conditions, the concentration of extraneuronal noradrenaline was estimated to be2.3 × 10 −6M. The EC 50 for the dilatatory effect of noradrenaline on the iris in vitro was2.1 × 10 −6M. We conclude that noradrenaline is present extraneuronally in the iris of the untreated rat in a concentration high enough to maintain a sympathetic tone. The possibility that this is a consequence of nonsynaptic neurotransmission is discussed.